1. Field of the Invention
This invention relates to polymers and, in particular, to the modification of the properties of organic polymers.
2. Art Background
Although the use of polymers in commercial products is widespread, polymers such a poly(vinyl chloride) (PVC) are rarely used without additives blended with the polymers to modify their properties. For example, it is generally necessary to include a plasticizer with the polymer to allow the polymer to be easily molded, extruded, or otherwise formed into a desired shape. It is also required for many applications that the polymer be flame retardant. Indeed, for some demanding applications, such as the use of PVC for telephone cord coatings, it is necessary that the polymer satisfy very strict flame retardancy standards, e.g., Underwriters Laboratory (UL) 62, Vertical-Wire (VW) 1, which, for example, ensures that the polymer is self-extinguishing before propagation of a flame initiates. Additionally for many applications, such as those involving consumer products, it is desirable that the polymer composition be clear, i.e., have a yellowness index as described in ASTM 1925 no greater than 12 at room temperature and no greater than 15 after 2 hours in a forced air oven at 150 degrees C.
Obviously, the numerous applications for which polymers are employed place correspondingly demanding strictures on polymer properties. However, the simultaneous satisfaction of these demands, e.g., flame retardancy, clarity, and desirable mechanical characteristics, is often extremely difficult to achieve. For example, a typical plasticizer for PVC involves the use of a phthalate. The use of these plasticizers, however, causes the product to exude and discolor. Similarly, a typical flame-retardant additive, antimony trioxide, substantially degrades the clarity of PVC. One system involving phosphate plasticizers, described in U.S. Pat. No. 4,346,145 issued Aug. 24, 1982, (which is hereby incorporated by reference) produces a degree of flame retardancy and desirable mechanical properties but causes some degradation in clarity which is not entirely desirable for some extremely demanding applications.
The additional requirement of long-term stability imposed by many applications even further increases the difficulty of formulating a suitable additive system. Long-term stability is necessary, for example, when the polymer during processing or in subsequent use is subjected to elevated temperatures. Again, additives, i.e., heat stabilizers, such as a diglycidyl ether of bisphenol-A, enhance the heat stability of PVC at the expense of humidity aging characteristics. To exacerbate the demands on an additive system, certain properties are generally adversely affected and rarely improved by the use of additives. Such affected properties include flexibility (as defined by ASTM D-746) and the compatibility of the polymer with other polymers.
Polymers such as PVC must be blended with many additives, such as a flame-retardant, a plasticizer, and a heat stabilizer. Generally, each additional additive substantially augments the price of the blended polymer. One additive often enhances one property but either degrades a second property, e.g., clarity, or degrades the efficacy of other additives. Additives also have the disadvantageous tendency to separate from the polymer and coat-processing equipment. It is, therefore, extremely difficult to produce a suitable additive system or to improve on a desirable additive system.